Summary of Work: Peroxisome Proliferators (PP) are known hepatocarcinogens in rats and mice. These compounds nclude environmental pollutants such as herbicides, plasticizers, and hypolipidemic drugs. Although many PP are hepato-carcinogenic in rodents (rats and mice), the mechanisms underlying this aspect of PP action as well as the human risks to these chemicals remain unclear. We have investigated the ability of Wyeth-14, 643 (WY), a PP and potent rodent carcinogen, to induce replicative DNA synthesis and to modulate the levels of PP activated receptor-alpha (PPARa) transcriptionally-dependent genes in primary rat hepatocyte (HPC) cultures and Hepatocyte/nonparenchymal cell (HPC/NPC) co-cultures. Cells were treated with WY and replicative DNA synthesis was quantitated using 3H-thymidine incorporation and specific mRNA transcript levels were determined by reverse-transcriptase polymerase chain reaction (RT-PCR). An increase in HPC replicative DNA synthesis was detected at 48 hours in both WY-treated HPC and HPC/NPC cocultures relative to controls. This increase was approximately 3 and 6 fold in HPC and HPC/NPC cultures, respectively, and wasa WY concentration- dependent. The levels of PPARa-transcriptionally dependent genes (cytochrome P450A1, acyl-CoA oxidase), and liver-fatty acid binding protein (L-FABP) transcripts were determined as indicators of PPARa activation. These transcripts increased dose-dependently at 48 hours in both HPC and HPC/NPC cultures up to 10 muM WY. We also investigated the mechanisms of inductionof replicative DNA synthesis by WY. WY-induced DNA synthesis was inhibited by 10-undecynoic acid (4504A1 inhibitor) in a dose-dependent manner, with complete inhibition achieved at 250 muM. Futher, WY-induced DNA synthesis was inhibited by indomethacin (cyclooxygenase inhibitor) in a dose-dependent manner with complete inhibiton observed at 10 muM.